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First & Second-Degree Equations

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Source: — Data Sufficiency |

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Re: First & Second-Degree Equations

by Brent@GMATPrepNow » Sun Mar 08, 2009 6:21 pm
BlindVision wrote:If rs IS NOT equal to 0, is 1/r + 1/s = 4?

1) r + s = 4rs

2) r = s

OA = A
I spent a half-hour trying to figure this one out. Please help :D
To begin, let's reword the question, by adding 1/r + 1/s to create one rational expression.
Find common denominator: s/rs + r/rs
Add numerators: (s+r)/rs
So, rather than ask "Does 1/r + 1/s = 4?" we can ask "Does (s+r)/rs = 4?"
Even better, we can cross multiply (s+r)/rs = 4/1 to get s+r = 4sr
So, another wording of the question can be "Does s+r = 4sr?"

(1) r + s = 4rs (SUFFICIENT)
(2) Knowing that r=s does not help us determine whether s+r = 4sr (INSUFFICIENT)
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Re: First & Second-Degree Equations

by logitech » Sun Mar 08, 2009 7:07 pm
I always start with rephrasing what is asked and trying to express it as simple as I can:

1/r + 1/s = 4

means: r+s/rs = 4 ; r+s=4rs ?

Statement 1) BINGO
Statement 2) INSUF

Choose (A)
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by BlindVision » Sun Mar 08, 2009 8:14 pm
B.H. & Logitech! Thank you... you guys helped me make more sense of it :D
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